The objects in the scene are shown according to z-depth, while still being drawn in the order of their priority: highest priority at the top of the list to the lowest priority at the bottom. The lowest level object is drawn first, the second to lowest level object is drawn next, and so on, until the top of the list.
To change the drawing order of objects:
The selected object name is highlighted in yellow.
When you move an object in the stack, a keyframe is added to the timebar.
Occasionally, a matte may obscure another object if the drawing order is unordered. If this happens, move the surface up and down in the stack until the drawing order is corrected, or click Z-Sort.